1. Field of the Invention
The present invention relates generally to PWM driven DC-DC converters, and relates more particularly to a current-fed isolated PWM DC-DC converter with reduced inductance and no deadtime.
2. Description of Related Art
Current-fed DC-DC converters have recently seen a resurgence in the number of applications in which they are used. In general, the term “current-fed” in an isolated DC-DC converter refers to the fact that the filter inductor of the converter is on the primary side. The voltage and current of the primary winding of the transformer in the DC-DC converter are determined by the load voltage and the source impedance, for example, the inductance of the primary side inductor. Current-fed topologies obtain noticeable advantages including immunity from transformer flux imbalance and a lack of an output inductor, which makes the current-fed topology useful for multi-output applications. Typical examples of isolated current-fed DC-DC converter topologies include current-fed full bridge topologies, fly back-current-fed push-pull converters, and their derivations.
One difficulty with conventional current-fed DC-DC converters is that the energy provided from the input DC source to the load is discontinuous over the entire operating period. In part of an operating period, the energy to the load is solely provided by the output capacitor. Accordingly, a larger inductor is often used in the current-fed DC-DC converter, and power densities are not necessarily optimal. A conventional current-fed full bridge DC-DC converter is illustrated in FIG. 1. The secondary side of transformer T, which provides the output of the converter, includes an output capacitor Co, which is appropriately sized and rated to handle the output voltage ripple. Output capacitor Co typically supplies energy to the load during discontinuous intervals of operation. Accordingly, capacitor Co has a relatively large rating, but less than optimal power density.
Conventional current-fed DC-DC converters driven by PWM signals have a deadtime interval. The term deadtime is defined to mean the time duration in an operating period that is used to obtain a regulated output voltage. During the deadtime, the energy transmission from an input DC source to an output load is not continuous and only provided, for example, by the output capacitor.
It would be desirable to obtain a current-fed DC-DC converter with reduced requirements for component ratings, while improving power density and avoiding deadtime inefficiencies.